Bandstop filters are designed to attenuate bands of unwanted signals which are input into the filter. A conventional bandstop filter, as seen in FIG. 1, generally includes an input 10 and an output 12 connected by interconnecting transmission lines 14. A number of resonators 16 are connected along the interconnecting transmission lines 14 between terminals 18 and ground. Each resonator has a characteristic capacitance denoted by capacitor 20 and a characteristic inductance denoted by inductor 22, which are chosen so that the resonators 16 resonate at the desired frequency with the desired bandwidth. At the resonant frequency, there is very low impedance between the terminal and ground, such that the signals at or near that frequency are attenuated. The electrical length of each interconnecting transmission line is generally chosen to be on the order of one-quarter wavelength or three-quarter wavelength of the resonant frequency for the resonators.
While it is possible to design such resonant circuits for any frequency, frequencies having relatively long wavelengths and relatively narrow stopbands may be problematic. The necessary characteristics of the capacitors and the inductors may be such that they are difficult to implement, may lead to structures which exhibit unacceptable signal losses outside of the desired stopband, or may fail to adequately attenuate signals in the stopband. The use of high-temperature superconducting structures in electromagnetic devices has been suggested because of their low resistances once cooled to below a critical temperature. By using superconductors in stopband filters, dissipation within a stopband can be increased without additional losses outside the stopband.
Superconductors have numerous drawbacks, however. First, the only high-temperature superconducting structures known are ceramics, and it may be difficult to connect those structures with other elements in a circuit. If a superconducting inductor 22 is used in FIG. 1, it may be difficult to connect that inductor 22 to a ground. Moreover, the necessity of cooling the superconducting structures makes it impractical to design circuits having large superconducting elements. For instance, it may be necessary to design a portion of a resonant element with a wavelength which is approximately one-half or one-quarter the wavelength of the resonant frequency. For applications in the high-frequency (HF) band, on the order of 3 to 30 megahertz, the long wavelength may make it impractical to use a superconducting structure.
In addition, it may be desirable for a user of a filter to change the resonant frequency of the resonators to adjust the stopband for a filter. When superconducting components are used, the filter will be housed in a cryostat to maintain low temperature, making adjustments to the filter difficult. In some cases it may be impractical, depending on the structure of the components, to adjust the frequency.